Balancing a Zoned System

Although the process for balancing a zoned system is similar to that of a constant volume system, balancing a zoned system can present a separate set of challenges and opportunities.

Balancing a constant volume system fixes the airflow to meet design conditions for the warmest days of summer. As the unit cycles on and off in rooms that share a similar load, adequate comfort is achieved. But where the sun travels across the sky under hot summer conditions, some rooms are built with exposures that require “conditional balancing” or zone dampers that alter the airflow or balance in a room to satisfy changing conditions throughout the day.

Balancing a zoned system requires a few more steps. Let's take a look at the zoned system balancing procedure and investigate the differences. The added work increases the labor 25% to 40% on a typical residential or light commercial project.

The first step is to inspect the system and compare the plan to what actually got built. This sounds really basic, but is critical for balancing a system. Check the usual components:

Did all the duct runs get installed?

Are all the end caps there?

Are the manual dampers open?

Are the supply registers delivering supply or return air volume?

Then check the added zoning components:

Is the bypass duct installed?

Is the control panel wired correctly?

Were all the dampers installed and connected?

Does each of the dampers operate freely when called to function?

Also, check the entire building for pressure imbalances that may be caused when dampers open and close. Each zone should have a return air duct that can handle the airflow under the zones highest airflow condition.

Start Balancing

To begin balancing, set all zoning dampers in open position. The better zoning systems have a switch in the control panel that allows the balancer to do this.

Set each thermostat in cooling mode at 55 degrees with the fan on to open all the zoning dampers 100%.

Let the system run until the coil is wet.

Take your initial airflow readings by measuring and recording the airflow at each register and grille.

Then, using the manual balancing dampers, balance each grille and register to the amount of airflow required under full cooling mode.

Up to this point, the balancing is similar to balancing a constant volume system.

Add together all the supply register airflows and check for 400 CFM per ton. If airflow is low, measure total external static pressure by measuring pressure before and after the air handler and adding the pressures together.

If needed, measure the pressure drop over the filter and the coil. Increase fan speed as needed. Then adjust the manual balancing dampers so each register gets the required airflow.

The accepted rule is plus or minus 10%.

Lock down and mark your manual balancing dampers. The system is balanced under constant volume conditions.

Do final testing that includes system temperatures, static pressures, blower motor and draw, and fan RPM and record these numbers on you balancing report.

Zone Balancing

Close down the largest zone in the system.

Next traverse the bypass duct to check the volume of bypass air.

Then by manipulating thermostat controls, open and close each of the zones in the system.

Test and record the zoned airflows and temperatures through the system under partially closed operating conditions.

Check for noise caused by excessive velocities.

As the zones open and close, measure the airflow passing through the bypass duct. Adjust it as necessary to assure quiet system operation and adequate temperatures through the heating and cooling equipment.

Check for excessive changes in total external static pressures or temperature drop or rise in the system.

Mark your final damper settings.

Complete your final equipment testing by measuring and recording full open pressures, temperatures, speed and electrical values, and complete the job as you would a constant air volume system.

Write an advisory statement to include with your final report describing the operation of the system with the largest zone closed. Include additional test numbers and observations you have made of the system. Make any design change or repair recommendations that may improve the quality of the system, if necessary.

Instruct the customer in proper use of the controls and help them understand what they can expect from their new system. Present a final bound copy of the balancing report with all the required paperwork, warranty information and service agreements.

Consider zoning as a tool to adapt balancing to the temperature requirements of certain rooms that have varying load conditions. As you design systems, be aware of rooms that may need more than just air balancing to keep them throughout the day and comfortable all year.

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